1. Field of the Invention
The present invention relates to an automatic tool change apparatus wherein numbers of tool holding devices stored in a magazine and numbers of tools held in the respective tool holding devices are correspondingly stored in a memory device, a tool commanded by numerical control data (NC data) is searched with its number in the memory device and indexed at a tool change position, a tool transfer device performs a tool change operation to move the commanded tool into the spindle and return the tool held in the spindle into an empty tool holding device, and the content of the memory device is rewritten at every tool change operation.
2. Description of the Prior Art
In a conventional automatic tool change apparatus capable of a random tool selection, the used tool is returned to a tool holding device holding a tool to be next used. Accordingly, if large and small tools are mixedly used, it may sometimes happen that the used tool cannot be returned. For example, assuming that a tool commanded by NC data is a small one and the used tool is a large one, the used tool cannot be returned, since if the used tool is returned to a tool holding device holding the small tool, the used tool interferes with tools held in adjacent tool holding devices.
For this reason, it has been the practice that the used tool is again returned to the tool holding device which held the same before its use, if large and small tools are mixedly used. However, in this system, two indexing operations are necessary at every tool change operation to index a tool holding device holding a tool commanded by NC data and to index a tool holding device into which the used tool is to be returned, which results in a longer tool change time.